Natural gas, which is primarily methane, and other gases, are liquefied under pressure for storage and transport. The reduction in volume that results from liquefaction permits containers of more practical and economical design to be used. Liquefaction is typically accomplished by chilling the gas through indirect heat exchange by one or more refrigeration cycles. Such refrigeration cycles are costly both in terms equipment cost and operation due to the complexity of the required equipment and the required efficiency of performance of the refrigerant. There is a need, therefore, for gas cooling and liquefaction systems having improved refrigeration efficiency and reduced operating costs with reduced complexity.
Liquefaction of natural gas requires cooling of the natural gas stream to approximately −160° C. to −170° C. and then letting down the pressure to approximately ambient. FIG. 1 shows typical temperature—enthalpy curves for methane at 60 bar pressure, methane at 35 bar pressure and a mixture of methane and ethane at 35 bar pressure. There are three regions to the S-shaped curves. Above about −75° C. the gas is de-superheating and below about −90° C. the liquid is subcooling. The relatively flat region in-between is where the gas is condensing into liquid. Since the 60 bar curve is above the critical pressure, there is only one phase present; but its specific heat is large near the critical temperature, and the cooling curve is similar to the lower pressure curves. The curve containing 5% ethane shows the effect of impurities which round off the dew and bubble points.
A refrigeration process is necessary to supply the cooling for liquefying natural gas, and the most efficient processes will have heating curves which closely approach the cooling curves in FIG. 1 to within a few degrees throughout their entire range. However, because of the S-shaped form of the cooling curves and the large temperature range, such a refrigeration process is difficult to design. Because of their flat vaporization curves, pure component refrigerant processes work best in the two-phase region but, because of their sloping vaporization curves, multi-component refrigerant processes are more appropriate for the de-superheating and subcooling regions. Both types of processes, and hybrids of the two, have been developed for liquefying natural gas.
Cascaded, multilevel, pure component cycles were initially used with refrigerants such as propylene, ethylene, methane, and nitrogen. With enough levels, such cycles can generate a net heating curve which approximates the cooling curves shown in FIG. 1. However, the mechanical complexity becomes overwhelming as additional compressor trains are required as the number of levels increases. Such processes are also thermodynamically inefficient because the pure component refrigerants vaporize at constant temperature instead of following the natural gas cooling curve and the refrigeration valve irreversibly flashes liquid into vapor. For these reasons, improved processes have been sought in order to reduce capital cost, reduce energy consumption and improve operability.
U.S. Pat. No. 5,746,066 to Manley describes a cascaded, multilevel, mixed refrigerant process as applied to the similar refrigeration demands for ethylene recovery which eliminates the thermodynamic inefficiencies of the cascaded multilevel pure component process. This is because the refrigerants vaporize at rising temperatures following the gas cooling curve and the liquid refrigerant is subcooled before flashing thus reducing thermodynamic irreversibility. In addition, the mechanical complexity is somewhat less because only two different refrigerant cycles are required instead of the three or four required for the pure refrigerant processes. U.S. Pat. No. 4,525,185 to Newton; U.S. Pat. No. 4,545,795 to Liu et al.; U.S. Pat. No. 4,689,063 to Paradowski et al. and U.S. Pat. No. 6,041,619 to Fischer et al. all show variations on this theme applied to natural gas liquefaction as do U.S. Patent Application Publication Nos. 2007/0227185 to Stone et al. and 2007/0283718 to Hulsey et al.
The cascaded, multilevel, mixed refrigerant process is the most efficient known, but a simpler, efficient process which can be more easily operated is desirable for most plants.
U.S. Pat. No. 4,033,735 to Swenson describes a single mixed refrigerant process which requires only one compressor for the refrigeration process and which further reduces the mechanical complexity. However, for primarily two reasons, the process consumes somewhat more power than the cascaded, multilevel, mixed refrigerant process discussed above.
First, it is difficult, if not impossible, to find a single mixed refrigerant composition which will generate a net heating curve closely following the typical natural gas cooling curves shown in FIG. 1. Such a refrigerant must be constituted from a range of relatively high and low boiling components, and their boiling temperatures are thermodynamically constrained by the phase equilibrium. In addition, higher boiling components are limited because they must not freeze out at the lowest temperatures. For these reasons, relatively large temperature differences necessarily occur at several points in the cooling process. FIG. 2 shows typical composite heating and cooling curves for the process of the Swenson '735 patent.
Second, for the single mixed refrigerant process, all of the components in the refrigerant are carried to the lowest temperature level even though the higher boiling components only provide refrigeration at the warmer end of the refrigerated portion of the process. This requires energy to cool and reheat these components which are “inert” at the lower temperatures. This is not the case with either the cascaded, multilevel, pure component refrigeration process or the cascaded, multilevel, mixed refrigerant process.
To mitigate this second inefficiency and also address the first, numerous solutions have been developed which separate a heavier fraction from a single mixed refrigerant, use the heavier fraction at the higher temperature levels of refrigeration, and then recombine it with the lighter fraction for subsequent compression. U.S. Pat. No. 2,041,725 to Podbielniak describes one way of doing this which incorporates several phase separation stages at below ambient temperatures. U.S. Pat. No. 3,364,685 to Perret; U.S. Pat. No. 4,057,972 to Sarsten, U.S. Pat. No. 4,274,849 to Garrier et al.; U.S. Pat. No. 4,901,533 to Fan et al.; U.S. Pat. No. 5,644,931 to Ueno et al.; U.S. Pat. No. 5,813,250 to Ueno et al; U.S. Pat. No. 6,065,305 to Arman et al.; U.S. Pat. No. 6,347,531 to Roberts et al. and U.S. Patent Application Publication 2009/0205366 to Schmidt also show variations on this theme. When carefully designed they can improve energy efficiency even though the recombining of streams not at equilibrium is thermodynamically inefficient. This is because the light and heavy fractions are separated at high pressure and then recombined at low pressure so they may be compressed together in the single compressor. Whenever streams are separated at equilibrium, separately processed and then recombined at non-equilibrium conditions, a thermodynamic loss occurs which ultimately increases power consumption. Therefore the number of such separations should be minimized. All of these processes use simple vapor/liquid equilibrium at various places in the refrigeration process to separate a heavier fraction from a lighter one.
Simple one stage vapor/liquid equilibrium separation, however, doesn't concentrate the fractions as much as may be accomplished using multiple equilibrium stages with reflux. Greater concentration allows greater precision in isolating a composition which will provide refrigeration over a specific range of temperatures. This enhances the process ability to follow the S-shaped cooling curves in FIG. 1. U.S. Pat. No. 4,586,942 to Gauthier and U.S. Pat. No. 6,334,334 to Stockmann et al. describe how fractionation may be employed in the above ambient compressor train to further concentrate the separated fractions used for refrigeration in different temperature zones and thus improve the overall process thermodynamic efficiency. A second reason for concentrating the fractions and reducing their temperature range of vaporization is to ensure that they are completely vaporized when they leave the refrigerated part of the process. This fully utilizes the latent heat of the refrigerant and precludes the entrainment of liquids into downstream compressors. For this same reason heavy fraction liquids are normally re-injected into the lighter fraction of the refrigerant as part of the process. Fractionation of the heavy fractions reduces flashing upon re-injection and improves the mechanical distribution of the two phase fluids.
As illustrated by U.S. Patent Application Publication No. 2007/0227185 to Stone et al., it is known to remove partially vaporized refrigeration streams from the refrigerated portion of the process. Stone et al. does this for mechanical reasons (not thermodynamic) and in the context of a cascaded, multilevel, mixed refrigerant process requiring two, separate, mixed refrigerants. In addition, the partially vaporized refrigeration streams are completely vaporized upon recombination with their previously separated vapor fractions immediately prior to compression.